Looks like I'm not the only one with the great idea of pledge auctions. There is a site, fundable.org, which implements the idea with paypal. How great! But this shows one problem with searching the net: Apparently they have been around since 2004, but although I have searched and searched for something like this, I haven't found it until now, and that by random chance (three or four links from boingboing). Not entirely random, I suppose, I was looking up some people with creative ideas, but nonetheless.
My problem was that I had only my own term for the concept (pledge auctions) and an understanding of what I meant by it. Neither could be put into a search engine - well, my own term would, but it didn't work because fundable.org calls them group auctions.